Devices with preset reverse conduction thresholds, such as Zener diodes, are used in many different applications as voltage limiters or references. They effect a limitation of the maximum voltage which can be applied between two nodes of a circuit or maintain a node at a preset voltage value with respect to a reference voltage. As is known in a Zener diode, when the reverse voltage applied between an anode terminal and a cathode terminal exceeds a specified threshold value Vz (Zener voltage), which typically can vary from a few volts to some tens or hundreds of volts, the diode goes into reverse conduction with a variable current and substantially constant voltage.
Zener diodes are generally made by exploiting the phenomenon of the nondestructive reverse breakdown of a PN junction. Commonly Zener diodes have a surface structure in which the PN junction is formed between a P type region and an adjacent N type region which extend from an upper surface of a semiconductor chip. Such Zener diodes have low series resistance and a voltage-current characteristic curve which exhibits a very sharp discontinuity at the threshold voltage. A drawback with Zener diodes of the surface type is that they exhibit drift phenomena in the long term which causes an increase in the series resistance and in the threshold voltage.
Zener diodes with buried structures are known, as illustrated in the partial sectional schematic view of FIG. 1. As is usual, the concentrations of the impurities of type N and P are indicated by adding the sign + or the sign - to the letters N and P to indicate a high or low concentration, respectively, of impurities. The letters N and P without the addition of + or - signs denote concentrations of intermediate value. The figure shows a device 100 having a preset reverse conduction threshold made inside an insulated region 105 of N type made in a semiconductor chip. A P type region 110 extends from an upper surface of the chip inside the insulated region 105. An N+ type region 115 extends from the same surface inside the region 110. A P+ type region 120 extends from the lower surface of the region 115 until it reaches, through the region 110, the insulated region 105. The P+ type region 120 defines with the N+ region 115 a buried PN junction. Conductive lines 125 and 130 in contact with surface areas of the regions, 110 and 115 respectively, form an anode electrode and cathode electrode, respectively, of the device 100.
The equivalent circuit of the device 100, as shown in the figure, comprises a Zener diode 135 formed by the buried PN junction described above, whose cathode terminal is connected to the electrode 130. A drawback of this structure is that the device 100 exhibits high series resistance due to the P type region 110, represented schematically by a resistor 140 connected between an anode terminal of the Zener diode 135 and the electrode 125.
Moreover, the P region 110 and the N+ region 115 form a further Zener diode 145 with the surface structure, the anode and cathode terminals of which are linked, respectively, to the electrodes 125 and 130. Using V.sub.b and V.sub.s to indicate the threshold voltages of the Zener diodes, 135 and 145 respectively, R the resistance of the resistor 140 and I the current which passes through the Zener diode 135 and the resistor 140, for the surface Zener diode 145 to be non-conducting it is necessary that the following condition be satisfied: EQU R.multidot.I+V.sub.b &lt;V.sub.s
i.e. EQU I&lt;(V.sub.s -V.sub.b)/R
In the case in which the threshold voltages of the two Zener diodes 135 and 145 have a similar value, this current is extremely small and hence uncontrollable.